Non-volatile memory arrays, such as erasable, programmable read only memory (EPROM) or flash memory arrays, or electrically erasable, programmable read only memory (EEPROM) arrays, require high positive or negative voltages to program and erase memory cells of the array. Typically, these voltages are higher than the voltage supplied (Vdd). Charge pumps are generally used to boost on-chip voltages above the supply voltage Vdd to reach the voltages required for programming or erasing.
A charge pump typically comprises cascaded stages that progressively boost the voltage to higher levels. The charge pump functions by progressively storing more charge on a capacitor which is part of a capacitor-diode combination, with several such stages being placed together in a network to obtain the desired increase in voltage. The diode functions to prevent discharge of the capacitor prior to placing the additional charge thereon.
Many applications require measuring the output current of the charge pump, for example, for measuring the program or erase current for flash cell applications. In the prior art, measuring the output current of the charge pump is typically accomplished by sensing or measuring internal signals of the charge pump. For example, for an on/off charge pump, the internal signal “oscillator_on” include reference information about the charge pump load—the ratio of the actual output current to the maximum output current (Iout/Iout_max). For a supply charge pump, the internal signal “supply clamp” may provide that reference information.
However, using these internal signals has disadvantages. The charge pump maximal output current Iout_max is a function of several parameters, such as but not limited to, Vdd, temperature and corner technology process. Since the ratio of the actual output current to the maximum output current is dependent upon these parameters, it is not possible to measure the absolute value of the output current (Iout).
Other methods have been used in the art to measure the output current, but they have disadvantages, such as requiring significant circuit board area and/or having relatively high power dissipation.